The present invention is directed to self scanned amorphous silicon integrated displays and in particular to displays employing an active bus to distribute signals across the display and having a column driver which exhibits reduced voltage stress.
Active matrix liquid crystal displays generate images by altering the polarization of individual picture elements using a liquid crystal material. The picture elements (pixels) are arranged in rows and columns. Image data is loaded into the liquid crystal display one row at a time. The rows of pixels are sequentially scanned in order to form image frames.
Each pixel in an active matrix display includes a thin film transistor (TFT). The thin film transistor receives video data from a column driver on the display when the display row containing the pixel is selected. The TFT stores the received video data onto the capacitance of the pixel.
One material which may be used to form active matrix liquid crystal displays (LCDs) is amorphous silicon. This material has the advantage that it may be fabricated at relatively low temperatures.
Because the TFTs or the pixels are fabricated from amorphous silicon. It is desirable to implement the peripheral circuitry, for example, the line scanners and column drivers using TFTs. It is difficult to design circuitry with TFTs, however, because they exhibit threshold drift. Threshold drift is a phenomenon where the gate to source voltage needed to turn on the transistor changes over time. In amorphous silicon TFTs, threshold drift occurs when a TFT is driven at a high duty cycle. Another problem with amorphous silicon active matrix displays and particularly to active matrix displays which use a chopped ramp signal, is the sensitivity of the display to slight variations across the screen in the value of the ramp signal to activate the individual pixel elements (pixels) of the display. These variations typically occur due to resistance in the distribution bus which applies the ramp signal across the columns of the display.
U.S. Pat. No. 5,670,979 to Huq et al. entitled "Dataline Drivers with Common Reference Ramp Display" discloses a column driver implemented with amorphous silicon technology. The column driver disclosed in this patent includes circuitry which adjusts the drive voltage of certain ones of the transistors to accommodate for threshold drift in these transistors. The disclosed circuit, however, drives transistors at a relatively high duty cycle, and thus undesirably reduces the expected lifetime of these transistors.